The present invention relates to a cryptography method on elliptic curve. Such a method is based on the use of a public key algorithm and can be applied to the generation of probabilistic digital signals of a message and/or to a key exchange protocol and/or to a message enciphering algorithm.
An algorithm for generating and verifying digital signatures consists in calculating one or more integers, in general a pair, known as the signature, and associated with a given message in order to certify the identity of a signature and the integrity of the signed message. The signature is said to be probabilistic when the algorithm uses a random variable in the generation of the signature, this random variable being secret and regenerated at each new signature. Thus the same message transmitted by the same user may have several distinct signatures.
Key exchange protocol and enciphering algorithms also use a secret random variable k generated at each new application of the algorithm.
Public key cryptography algorithms on elliptic curves are being used more and more. Such an algorithm is based on the use of points P(x,y) on a curve E satisfying the equation:y2+xy=x3+ax2+b, with a and b two elements of a finite field.
Addition or subtraction operations are performed on the points P of the curve E. The operation consisting in adding k times the same point P is called the scalar multiplication of P by k, and corresponds to a point C on the elliptic curve defined by C(x′,y′)=k·P(x,y).
An example of such an algorithm can be illustrated by the ECDSA (from the English Elliptic Curve Digital Standard Algorithm), which is an algorithm for generating and verifying probabilistic digital signatures.
The parameters of the ECDSA are:                E, an elliptic curve defined on the set Zp, the number of points on the curve E being divisible by a large prime number N, in general N>2160,        P(x,y), a given point on the elliptic curve E.        
The secret key d is a randomly fixed number between 0 and N−1, and the public key Q is related to d by the scalar multiplication equation Q(x1,y1)=d·P(x,y).
Let m be the message to be sent. The ECDSA signature of m is the pair of integers (r,s) included in the range [1, N−1] and defined as follows:                let k be a random number chosen in the range [1, N−1], k being a random variable regenerated at each signature;        calculation of the point C obtained by the scalar multiplication C(x′,y′)=k·P(x,y);        r=x′ mod N;        s=k−1(h(m)+d·r) mod N;        
with h(m) the result of the application of a hash function h, which is a pseudo-random cryptographic function, to the initial message m.
The verification of the signature is performed, using public parameters (E, P, N, Q), as follows:
Intermediate calculations are carried out:                w=s−1 mod N;        u1=h(m)·w mod N;        u2=r·w mod N;        An addition and scalar multiplication operation is performed by calculating the point on the curve E corresponding to u1P+u2Q=(x0,y0)        
It is checked whether v=x0 mod N Υ r.
If this equality is true, the signature is authentic.
The generation of the signature (r,s) was performed with the secret key d and a secret random number k different for each signature, and its verification with the parameters of the public key. Thus anyone can authenticate a card and its bearer without holding its secret key.
The cost of execution of such a signature algorithm on elliptic curve is directly related to the complexity and speed of the scalar modification operation for defining the point C=k·P.
Improvements to the cryptography method on elliptic curves have been developed in order to facilitate and accelerate this scalar multiplication operation. In particular, the article by J. A. Solinas “An Improved Algorithm for Arithmetic on a Family of Elliptic Curves”, which appeared in Proceedings of Crypto'97, Springer Verlag, describes one possible improvement.
In order to accelerate the method for calculating a scalar multiplication in the context of an algorithm on elliptic curve E, it has thus been envisaged working on a particular family of elliptic curves, known as abnormal binary elliptic curves or Koblitz curves, on which a particular operator is available, known as a Frobenius operator, making it possible to calculate the scalar multiplication operations more quickly.
The Koblitz curves are defined on the mathematical set GF(2n) by the equation:y2+xy=x3+ax2+1 with aε{0,1]
The Frobenius operator T is defined as:τ[P(x,y)]=(x2,y2) with the equation τ2+2=(−1)1−aτ
Applying the operator τ to a given point P on the elliptic curve E constitutes a quick operation since the work is done in the mathematical set GF(2n), n being the size of the finite field, for example n=163.
In order to facilitate the calculation of the scalar multiplication C(x1,y1)=k·P(x,y), the integer k is decomposed so as to amount to addition and subtraction operations. In this way the non-adjacent form of the integer k is defined by the NAF (from the English Non-Adjacent Form), which consists in writing an integer k in the form of a sum:k=Σ(i=0 to 1−1)ei2i with eiε{−1, 0, 1} and 1≅n.
In the case of a Koblitz elliptic curve, the NAF can be expressed by means of the Frobenius operator:k=Σ(i−0 to 1) eiτi.
Thus the operation of scalar multiplication of P by k amounts to applying the Frobenius operator to the point P, which is easy and rapid.
In addition, the calculation of the scalar multiplication k·P can be accelerated further by the precalculation and storage of a few pairs (ki, Pi=ki·P), these pairs advantageously being able to be stored in the memory of the device implementing the signature algorithm. In fact P forms part of the public parameters of the key of the signature algorithm.
For a random variable k of 163 bits, it is thus possible, by storing 42 scalar multiplication pairs (ki,Pi), to reduce the number of addition/subtraction operations to 19 instead of 52 without any precalculation.
The object of the present invention is a cryptography method on elliptic curve which makes it possible to reduce the number of additions of the scalar multiplication still further.